The production of closed-cell foams with linear polypropylene (“PP”) is difficult because the PP melt does not exhibit strain hardening under extensional flow. The lack of strain hardening leads to thinner cell walls during bubble expansion in the foaming process, coalescence of bubbles, and the formation of open-cell foams. Previous efforts to modify linear PP by introduction of long chain branching through reactive extrusion with organic peroxides such as peroxydicarbonates resulted in increased strain hardening, but the level of strain hardening alone was not sufficient to produce closed-cell foams. And further, it has been found that strain hardening alone is not sufficient, that sufficient melt strength as well as shear thinning is also necessary to achieve a desirable foam using PP. What is needed is a polypropylene that has sufficient melt strength and strain hardening to form commercially viable foamed articles.
Relevant publications include M. H. Wagner et al., “The strain-hardening behaviour of linear and long-chain-branched polyolefin melts in extensional flows,” in 39 RHEOL. ACTA 97-109 (2000); R. P. Lagendijk et al., in “Peroxydicarbonate modification of polypropylene and extensional flow properties,” in 42 POLYMER 10035-10043 (2001); P. Spitael et al., in “Strain hardening in polypropylenes and its role in extrusion foaming,” in 44(11) POLY. ENG. & SCI. 2090-2100 (2004); K. Jayaraman et al., “Entangling additives enhance polypropylene foam quality,” in SPE PLASTICS RESEARCH ONLINE (2011); P. Iacobucci, “High melt strength polypropylene through reactive extrusion with Perkadox 24L,” SPE POLYOLEFINS CONFERENCE, Houston, Tex. (February 2004); H. Pol et al., “Microstructure and rheology of high-melt-strength poly-(propylene) impact copolymer,” in SPE PLASTICS RESEARCH ONLINE (2014); M. Ratzsch et al., 27 PROG. POLYM. SCI. 27 1195 (2002); and N. Spisakova et al., in 15 J. MACRM. SCI. & APP. CHEM. 37 (2000); EP 2679630 A1; EP 2000504 A1; U.S. Pat. Nos. 5,047,485; 5,416,169; 5,883,151; 6,956,067; 6,875,826 A1; U.S. Pat. Nos. 6,573,343; 6,350,828; 6,323,289; 8,153,745; 9,068,030; 9,200,095; US 2002/0043643; US 2003/0157286 A1; US 2012/245302; WO 1994/005707 A1; and WO 2014/070386.